1. Field of the Invention
The present invention relates to a needle-bar driving device of a sewing machine.
2. Related Art Statement
A sewing machine has a needle-bar driving device. When a needle bar supporting at a lower end thereof a sewing needle is oscillated in the axial direction thereof by the needle-bar driving device, the sewing needle with a needle thread penetrates a work cloth, so that a loop of the needle thread is locked with a bobbin thread to form a stitch on the work cloth.
There is known a needle-bar driving device which is capable of carrying out "skip stitching" for a pattern sewing or a basting sewing, by reducing the number of stitches formed in a unit length on a work cloth fed by a feeding device, as compared with that for normal stitching or sewing, and thereby forming lengthened stitches. An example of such a needle-bar driving device is disclosed in Japanese Patent Application laid open for opposition under Publication No. 57(1982)-35675. In the prior device, a needle bar is connected, via an engagement member and an engagement pin which are engageable with each other, with a guide bar which is oscillated in the axial direction thereof by a needle-bar crank rod. In the normal sewing, the engagement member and engagement pin are engaged with each other, so that the oscillation of the crank rod is transmitted to the needle bar via the guide bar. Consequently, the needle bar is oscillated to form stitches on a work cloth. Meanwhile, in the skip stitching, the engagement member and engagement pin are disengaged from each other, so that the needle bar is independent of the oscillation of the guide bar. Consequently, the needle bar remains stopped and held at a predetermined position, and no stitch is formed on the work cloth.
In the prior needle-bar driving device which carries out the skip stitching with the needle bar being held at a predetermined position, however, a needle thread slides through the eye of the sewing needle supported by the needle bar, when the work cloth is fed by the feeding device. There arises a problem that the work cloth is dragged by the needle thread due to the friction between the thread and the inner surfaces of the needle eye. In particular, in the event that the feeding speed of the work cloth is so high, or in the event that the needle bar is stopped at a predetermined position so distant from the work cloth, the problem is worsened.
It is possible that the skip stitching be carried out without stopping a needle bar, but by changing the lower dead position of the needle bar, i.e., position where the needle bar is nearest to a work cloth during the axial-direction oscillation thereof, to a new lower dead position where a sewing needle supported by the needle bar cannot stick in the work cloth or where the sewing needle can stick in the work cloth but cannot form a stitch on the work cloth. However, there has conventionally been no needle-bar driving device which is capable of changing the lower dead position of a needle bar.
There are some additional problems for the reason that the lower dead position of a needle bar cannot be changed by the conventional needle-bar driving devices.
First, in the event that a boring operation is performed on a work piece by utilizing a boring blade fixed to the lower end of a needle bar in place of a sewing needle, it is preferred that the depth of a cut formed in the work piece by the blade be changeable depending upon the nature and/or thickness of the work piece. This would be achieved if the lower dead position of the needle bar could be changed.
Second, in the event that a marking operation is performed on a work piece by causing a sewing needle bearing no thread to stick in the work piece to produce marks or holes as a pattern for embroidering by hand, it is preferred that the holes produced in the work cloth by the needle be not excessively large. In particular, in the case where the marking operation is performed on a work piece having a greater resistance, such as a suede, buckskin, or artificial leather, a larger amount of heat is adversely generated due to the friction between the sewing needle and the work piece. In order to reduce the heat, it has conventionally been practiced to lower the rate or speed of the marking operation. However, these problems could easily be solved by changing the lower dead position of the needle bar.
Third, the normal sewing has been carried out by oscillating a sewing needle by an unnecessarily large amplitude. In order only to form stitches on a work cloth, it is not necessary to retract the sewing needle away from the work piece by so great a distance. The upper dead position of a needle bar may be established at a considerably low position above the work cloth. On the other hand, for mounting a work cloth on a sewing machine, it is preferred to establish the upper dead position of the needle bar at a considerably high position for avoiding the interference of the sewing needle with the work cloth being mounted. Thus, the conventional needle-bar driving devices oscillate a needle bar by an unnecessarily large amplitude, resulting in producing excessively large operational noise and having no room to increase the operational speed of the devices.